Reperfusion injury

Reperfusion (reoxygenation) injury is the tissue damage caused when blood supply returns to the tissue after a period of ischemia or lack of oxygen (anoxia, hypoxia). The absence of oxygen and nutrients from blood during the ischemic period creates a condition in which the restoration of circulation results in inflammation and oxidative damage through the induction of oxidative stress rather than restoration of normal function.

Reperfusion of ischemic tissues is often associated with microvascular injury, particularly due to increased permeability of capillaries and arterioles that lead to an increase of diffusion and fluid filtration across the tissues. Activated endothelial cells produce more reactive oxygen species but less nitric oxide following reperfusion, and the imbalance results in a subsequent inflammatory response.[1] The inflammatory response is partially responsible for the damage of reperfusion injury. White blood cells, carried to the area by the newly returning blood, release a host of inflammatory factors such as interleukins as well as free radicals in response to tissue damage.[2] The restored blood flow reintroduces oxygen within cells that damages cellular proteins, DNA, and the plasma membrane. Damage to the cell's membrane may in turn cause the release of more free radicals. Such reactive species may also act indirectly in redox signaling to turn on apoptosis. White blood cells may also bind to the endothelium of small capillaries, obstructing them and leading to more ischemia.[2] Another hypothesis would be that normally, tissues contain free radical scavengers to avoid damage by oxidizing species normally contained in the blood. Ischemic tissue would have a decrease function of these scavengers because of cell injury. Once blood flow is reestablished, oxygen species contained in the blood will damage the ischemic tissue because the function of the scavengers is decreased.

Reperfusion injury plays a part in the brain's ischemic cascade, which is involved in stroke and brain trauma. Similar failure processes are involved in brain failure following reversal of cardiac arrest;[3] control of these processes is the subject of ongoing research. Repeated bouts of ischemia and reperfusion injury also are thought to be a factor leading to the formation and failure to heal of chronic wounds such as pressure sores and diabetic footulcers.[4] Continuous pressure limits blood supply and causes ischemia, and the inflammation occurs during reperfusion. As this process is repeated, it eventually damages tissue enough to cause a wound.[4]

In prolonged ischemia (60 minutes or more), hypoxanthine is formed as breakdown product of ATP metabolism. The enzyme xanthine dehydrogenase acts in reverse, that is as a xanthine oxidase as a result of the higher availability of oxygen. This oxidation results in molecular oxygen being converted into highly reactive superoxide and hydroxylradicals. Xanthine oxidase also produces uric acid, which may act as both a prooxidant and as a scavenger of reactive species such as peroxynitrite. Excessive nitric oxide produced during reperfusion reacts with superoxide to produce the potent reactive species peroxynitrite. Such radicals and reactive oxygen species attack cell membrane lipids, proteins, and glycosaminoglycans, causing further damage. They may also initiate specific biological processes by redox signaling.

An intriguing area of research demonstrates the ability of a reduction in body temperature to limit ischemic injuries. This procedure is called therapeutic hypothermia, and it has been shown by a number of large, high-quality randomised trials to significantly improve survival and reduce brain damage after birth asphyxia in newborn infants, almost doubling the chance of normal survival. For a full review see Hypothermia therapy for neonatal encephalopathy.

However, the therapeutic effect of hypothermia does not confine itself to metabolism and membrane stability. Another school of thought focuses on hypothermia’s ability to prevent the injuries that occur after circulation returns to the brain, or what is termed reperfusion injuries. In fact an individual suffering from an ischemic insult continues suffering injuries well after circulation is restored. In rats it has been shown that neurons often die a full 24 hours after blood flow returns. Some theorize that this delayed reaction derives from the various inflammatory immune responses that occur during reperfusion.[7] These inflammatory responses cause intracranial pressure, pressure which leads to cell injury and in some situations cell death. Hypothermia has been shown to help moderate intracranial pressure and therefore to minimize the harmful effect of a patient’s inflammatory immune responses during reperfusion. Beyond this, reperfusion also increases free radical production. Hypothermia too has been shown to minimize a patient’s production of deadly free radicals during reperfusion. Many now suspect it is because hypothermia reduces both intracranial pressure and free radical production that hypothermia improves patient outcome following a blockage of blood flow to the brain.[8]

NEW UPDATE/NOTE: Recent studies done in 2014, the CIRCUS and CYCLE studies (published in September 2015 and February 2016 respectively) looked at the use of cyclosporin as a one time IV dose given right before perfusion therapy (PCI). Both studies found there is no statistical difference in outcome with cyclosporin administration.[10][11]

The opening of the MPT pore leads to the inrush of water into the mitochondria, resulting in mitochondrial dysfunction and collapse. Upon collapse, the calcium is then released to overwhelm the next mitochondria in a cascading series of events that cause mitochondrial energy production supporting the cell to be reduced or stopped completely. The cessation of energy production results in cellular death. Protecting mitochondria is a viable cardioprotective strategy.[14]

Cyclosporin has been confirmed in studies to inhibit the actions of cyclophilin D, a protein which is induced by excessive intracellular calcium flow to interact with other pore components and help open the MPT pore. Inhibiting cyclophilin D has been shown to prevent the opening of the MPT pore and protect the mitochondria and cellular energy production from excessive calcium inflows.[13]

In 2008, an editorial in the New England Journal of Medicine called for more studies to determine if cyclosporin can become a treatment to ameliorate reperfusion injury by protecting mitochondria.[14] To that end, in 2011 the researchers involved in the original 2008 NEJM study initiated a phase III clinical study of reperfusion injury in 1000 myocardial infarction patients in centers throughout Europe. Results of that study were announced in 2015 and indicated that "intravenous ciclosporin did not result in better clinical outcomes than those with placebo and did not prevent adverse left ventricular remodeling at 1 year".[15]

Note: This same process of mitochondrial destruction through the opening of the MPT pore is implicated in making traumatic brain injuries much worse.[16] Ciclosporin is currently in a phase II/III (adaptive) clinical study in Europe to determine its ability to ameliorate neuronal cellular damage in traumatic brain injury.

TRO40303 is a new cardioprotective compound that was shown to inhibit the MPT pore and reduce infarct size after ischemia-reperfusion. It was developed by Trophos company and currently is in Phase I clinical trial.[17]

A series of 2009 studies published in the Journal of Cardiovascular Pharmacology suggest that Metformin may prevent cardiac reperfusion injury by inhibition of Mitochondrial Complex I and the opening of MPT pore and in rats.[22][23]